This invention concerns a method to guide the strip between the stands in a rolling mill finishing train and the relative device to be installed between the stands.
The invention is applied in hot rolling mills, in particular but not exclusively in finishing trains for rolling thin strips.
A conventional inter-stand device in a finishing train for strip usually comprises a guide device placed at the outlet of the stand upstream, a looper unit and a guide device placed at the inlet to the stand downstream.
The inter-stand device, and particularly the guide devices, have the function of transporting the leading end of the strip from one stand to the next stand, preventing the strip from hitting the inlet as it enters and the consequent accordion-like "buckling".
The inter-stand device is also intended to prevent the trailing end of the strip, when it comes out of the upstream stand, going to damage the working rolls of the stand with vertical, "tail-wagging" movements.
The looper unit, apart from cooperating with the guide devices in the above-mentioned functions, is also intended to generate and measure a desired loop on the strip during the rolling process, making it possible to adjust the speed of the rolls in the stand in order to maintain a flow of material as steady as possible between the upstream stand and the downstream stand.
The guide devices can be adjusted in height in order to align, with a certain tolerance, the pass-line of the guides with the tangent wire steel of the respective lower working rolls of the upstream and downstream stands.
This alignment allows the inter-stand device to be adapted to the variations in size of the working rolls and back-up rolls of the rolling stands.
The guide devices and the looper unit are moreover associated with moving units which make it possible to change the working and back-up rolls.
In devices known to the state of the art, the looper roller is an idler roller and is made to rotate by the strip itself.
When the strip enters the stand, in the state of the art, the looper roller is kept in a lowered position so as not to interfere with the pass-line of the guide devices.
Once the leading end of the strip has entered the downstream stand, the looper roller is raised and brought into contact with the strip which starts it rotating.
The lifting and lowering movement of the looper roller is normally generated by an electric motor or by a hydraulic roll; the looper roller itself is controlled, if necessary with a feedback circuit, to maintain the drawing action between stands at substantially constant values.
This function makes it possible to prevent the formation of shrinkages and variations in the width of the strip being rolled which derive from a non-uniform drawing action.
In the case of thin strip being rolled, with a thickness of less than 1.2 mm, the configuration of the inter-stand device as described above involves a plurality of disadvantages.
If the final product is to have a thickness of 1.2 mm, the inter-stand device upstream of the last stand in the finishing train operates on a thickness of about 1.5 mm.
Until this limit value, the bending rigidity of the strip is such as to allow it to pass through the inter-stand device without too much difficulty even at speeds of about 9.div.10 meters per second.
For thicknesses of less than 1.5 mm, because of the friction generated by the strip slipping on the carrying surfaces of the guides and the looper roller, and because of the reduced bending rigidity, which varies in inverse proportion to the cubic power of the thickness, the transfer of the leading end of the strip is difficult, there are dangers of "buckling" and overlapping of the strip inside the inter-stand device.
Moreover, there is also the problem of the impact between the strip passing through and the looper roller, which is stationary at first; this impact can cause slipping which can damage the surface of the strip.
JP-A-3-81010 discloses a looper equipped with a roller which, when the strip enters the device, is rotating at a greater speed than that of the strip and, when the trailing end is passing through, is made to rotate in the inverse direction.
These differences in speed cause the looper to slip on the strip, damaging both the strip and itself, and this solution is therefore unacceptable.
JP-A-61255714 discloses a pre-heating system of the strip between two stands.
This prior art document includes two rollers (11, 26), which are not powered; they support the strip and have a function of strip tension detection.
Therefore, JP'714 deals with problems which are different from those addressed by this invention (see also the arm 18), as it deals with the need to provide a correct transit line, and to allow the strip to pass inside the furnace.
Nor are the embodiments therein disclosed suitable to be applied to this invention.